The ornate, pinecone-shaped towers of Angkor Wat in Cambodia float above a vast temple complex of shrines, pools, houses, and a perfectly square moat. Today, only a small number of monks remain within the temple walls. The remaining structures have been reclaimed by trees whose roots wind around the stone like cellulose tentacles. Archaeologists have long wondered what life was like here when Angkor was the cosmopolitan heart of the Khmer Empire in the 12th and 13th centuries. Why did so many people abandon this place in the 15th century, never to return?

Unlike a majority of archaeological endeavors, the answers didn't come from digging up the ground. Instead, our first glimpse of Angkor as it once was came just a few years ago from a sophisticated laser scanning machine mounted on a helicopter.

Invisible city

For centuries, the Angkor region's wealth of artifacts drew looters, archaeologists, and looter-archaeologists. They focused their attention, both good and ill, on Angkor Wat and a few other nearby moated temple complexes. Based on those ruins, the first European explorers to encounter Angkor in the 19th century assumed Khmer urbanites lived in what were basically moated cities of a few thousand people. These European explorers thought Angkor Wat was something like a medieval walled city in Europe, which typically held fewer than 10,000 people. They explained all the moated complexes in the Angkor area by suggesting that maybe the royal family and their people were moving from one moated city to the next over time. But as archaeologists learned more in the intervening century, something about those population numbers seemed off. Beyond the moated cities were vast canal systems and reservoirs hinting at something bigger.

Unfortunately, most of Angkor had become a tangle of jungles and small farms by the 20th century. There was little evidence of medieval settlements beyond the moats' precise edges. Even if explorers were willing to hack through the dense growth, there was little to find. In a Khmer city, only the temples were made from stone. Everything else was built from perishable materials like wood. All that remained of Angkor's homes and other non-religious structures were the elevated clay mounds of their foundations, which had been designed to prevent flooding during Cambodia's intense wet season. Most of the city's dramatic waterworks for flood runoff and water storage had been reduced to pits and troughs in the Earth. It was practically impossible to identify a medieval Angkorian house deep within the jungle.

All that changed when airborne LiDAR (for "Light Imaging, Detection, And Ranging") came into common use for mapping in the early 2000s. Archaeologists working in Cambodia immediately seized on it. By scattering light off the surface of the planet, LiDAR systems can produce maps with accuracy down to the centimeter even if the ground is covered in heavy vegetation. The system is ideal for a place like Angkor, where the city's remains are cloaked in vegetation and characterized almost entirely by elevated or depressed plots of ground.

With funding from the National Geographic Society and European Research council, archaeologist Damian Evans and his colleagues conducted broad LiDAR surveys of Angkor in 2012 and 2015. The team's mapping rig consisted of a Leica ALS70 HP LiDAR instrument mounted in a pod attached to the right skid of a Eurocopter AS350 B2 helicopter alongside a 60 megapixel Leica RCD30 camera. It was as if an invisible city suddenly appeared where only overgrowth and farmland existed before. For the first time in centuries, people could discern Angkor's original urban grid. And what they saw changed our understanding of global history.

Archaeological researcher Piphal Heng, who studies Cambodian settlement history, told Ars that the LiDAR maps peeled back the forest canopy to reveal meticulous grids of highways and low-density neighborhoods of thousands of houses and pools of water. There was "a complex urban grid system that extended outside the walls of Angkor Thom and other large temple complexes such as Angkor Wat, Preah Khan, and Ta Prohm," he said. With the new data, scientists had solid evidence that the city of Angkor sprawled over an area of at least 40 to 50 square km. It was home to almost a million people. The scattered, moated complexes like Angkor Wat and Angkor Thom were merely the most enduring features of what we now know was the biggest city on Earth during the 12th and 13th centuries.

This aerial photo shows what Angkor Wat looks like today, surrounded by vegetation and a few areas of modern farms and homes. None of the vast city grid from 800 years ago is visible.

Charles J Sharp

In the LiDAR map, you can clearly see the central urban grid of Angkor extending from Angkor Thom (top left) and Angkor Wat (bottom left).

LiDAR courtesy of KALC

Here you can see the areas covered by the LiDAR surveys in 2012 and 2015.

Background elevation courtesy of NASA SRTM.

A map of the greater Angkor area, showing the extent of the urban sprawl revealed by LiDAR.

The LiDAR rig was a Leica ALS70 HP instrument, mounted in a pod attached to the right skid of a Eurocopter AS350 B2 helicopter along with a 60 megapixel Leica RCD30 camera.

Francisco Goncalves

The ruins of Preah Khan of Kompong Svay covered with forest. An urban network was revealed by the lidar imagery around this temple.

Francisco Goncalves

From legend to reality

The city of Angkor has its origins in the ninth century during the reign of Jayavarman II. He unified large parts of Southeast Asia by establishing the Khmer Empire across regions we know today as Cambodia, Thailand, Vietnam, and Laos. Inscriptions on temple walls at Sadok Kok Thom in Thailand describe how he established a city called Hariharalaya, located near Siem Reap in the Angkor area. But the inscriptions also say that Jayavarman II declared himself a supreme ruler or "god-king" in a lavish Hindu ceremony held at his residence on Kulen Mountain in a city called Mahendraparvata. Accounts of the Kulen Mountain phase in Jayavarman's life are so sparse and fantastical that debates have raged among scholars about whether he actually lived in Mahendraparvata at all.

To find out more, archaeologists targeted Kulen Mountain in their latest LiDAR survey. Last month, Evans published some of the first results from this 2015 survey in the Journal of Archaeological Science. Royal Academy of Cambodia archaeologist Kaseka Phon explained to Ars via e-mail that the LiDAR has uncovered an Angkor-like city grid at the abandoned city of Mahendraparvata on Kulen Mountain. Plus, the LiDAR "shows not only features of the construction, but also water features" that are clearly versions of Angkor's incredible water management facilities. The new survey revealed massive stone quarries, now filled in, that produced the rock used to build some of the temples of Angkor. Kulen Mountain's role in the birth of the Khmer Empire is no longer a legend—it's an established historical fact.

This transformation of legend into fact has been a theme of the LiDAR surveys. Angkor's huge population is described in temple inscriptions and reports written by Chinese travelers who visited the city during the 12th century reign of King Suryavarman II, who built Angkor Wat. But historical sources are often exaggerated or incomplete. Plus, it was difficult for Western researchers to believe that the Khmer Empire's great city was home to almost a million people, dwarfing European cities of the same era. Now, such facts are impossible to deny.